Conventionally, known is a battery pack with a built-in overcurrent protection circuit for protecting the battery from an overcurrent by detecting the current flowing to the battery and, if the current becomes greater than the setting current, turning OFF the semiconductor switching element that is connected to the battery in series and blocking the current (for example, refer to Patent Document 1).
Meanwhile, with respect to the power consumption of a load circuit in electrical equipment that operate by receiving a power supply from the battery pack; for instance, electronic devices such as portable personal computers, digital cameras and cell phones, and vehicles such as electrical vehicles and hybrid cars, its maximum value is specified, and the battery pack is designed to enable the supply of such maximum power consumption.
Specifically, for example, when using a lithium ion secondary battery with the output voltage per cell being within the range of 3.5 V to 4.2 V, even if the output current value is the same, the output power will decrease as the output voltage becomes lower. Thus, with a battery pack, if the current flowing to the secondary battery; that is, if the output current of the secondary battery becomes greater than the setting current, the current is blocked so as to protect the secondary battery from an overcurrent.
Accordingly, the protection circuit of a secondary battery is set with a setting current for protection from an overcurrent to enable the supply of the maximum power consumption required by the electrical equipment in the available output voltage lower limit value.
Meanwhile, if an abnormality such as a short circuit malfunction occurs in the load circuit that receives the power supply from the secondary battery, there are cases where the power consumption of the load circuit; that is, the output power of the secondary battery increases and exceeds the specified maximum power consumption. In the foregoing case, if supply of power to the load circuit is continued, the load circuit may generate heat.
Nevertheless, as described above, with the method of blocking the current in cases where the output current of the secondary battery becomes greater than the setting current, if the output voltage of the secondary battery is low, the output current will exceed the setting current as a result of the power consumption of the load circuit exceeding the specified maximum power consumption, and it is thereby possible to block the power supply. However, if the output voltage of the secondary battery is high, even if the output current falls below the setting current, there are cases where the output power of the secondary battery exceeds the specified maximum power consumption.
Thus, with the method of blocking the current in cases where the output current of the secondary battery becomes greater than the setting current, there is a drawback in that it is difficult to protect the load circuit in cases where an abnormality occurs in the power consumption of the load circuit that receives the power supply from the secondary battery.    Patent Document 1: Japanese Patent Application Publication No. 2001-286068